docs: def Peclet_heat(V, L, rho=None, Cp=None, k=None, alpha=None):
      r'''Calculates heat transfer Peclet number or `Pe` for a specified velocity
      `V`, characteristic length `L`, and specified properties for the given
      fluid.
  
      .. math::
          Pe = \frac{VL\rho C_p}{k} = \frac{LV}{\alpha}
  
      Inputs either of any of the following sets:
  
      * V, L, density `rho`, heat capacity `Cp`, and thermal conductivity `k`
      * V, L, and thermal diffusivity `alpha`
  
      Parameters
      ----------
      V : float
          Velocity [m/s]
      L : float
          Characteristic length [m]
      rho : float, optional
          Density, [kg/m^3]
      Cp : float, optional
          Heat capacity, [J/kg/K]
      k : float, optional
          Thermal conductivity, [W/m/K]
      alpha : float, optional
          Thermal diffusivity, [m^2/s]
  
      Returns
      -------
      Pe : float
          Peclet number (heat) []
  
      Notes
      -----
      .. math::
          Pe = \frac{\text{Bulk heat transfer}}{\text{Conduction heat transfer}}
  
      An error is raised if none of the required input sets are provided.
  
      Examples
      --------
      >>> Peclet_heat(1.5, 2, 1000., 4000., 0.6)
      20000000.0
      >>> Peclet_heat(1.5, 2, alpha=1E-7)
      30000000.0
  
      References
      ----------
      .. [1] Green, Don, and Robert Perry. Perry's Chemical Engineers' Handbook,
         Eighth Edition. McGraw-Hill Professional, 2007.
      .. [2] Cengel, Yunus, and John Cimbala. Fluid Mechanics: Fundamentals and
         Applications. Boston: McGraw Hill Higher Education, 2006.
      '''
      if rho and Cp and k:
          alpha =  k/(rho*Cp)
      elif not alpha:
          raise Exception('Either heat capacity and thermal conductivity and\
          density, or thermal diffusivity is needed')
      return V*L/alpha